A conventional fuel cell electric power generating system is of the configuration shown in FIG. 4 as described in Japanese Patent Laid-Open Gazette No. HEI3-257762. That is, such a system includes a reformer 1 of producing hydrogen-rich gas from a source gas, a burner 2a as heating means 2 of heating the reformer 1, nitrogen equipment 16 located upstream of and connected to the reformer 1 via a nitrogen supply pipe 14 and a shut-off valve 15, and a fuel cell 9 located downstream of and connected to the reformer 1 via a reformed gas supply pipe 17 for generating electric power by reacting oxygen contained in air with hydrogen produced, the downstream side of hydrogen electrode 9a of the fuel cell 9 being connected to the burner 2a via an exhaust hydrogen connecting pipe 12. It is to be noted that the entire disclosure of Patent Laid-Open Gazette No. HEI3-257762 is incorporated herein by reference in its entirety.
When the electric power generating operation of a general fuel cell electric power generating system is to be stopped, the supply of the source gas is stopped first. At this time the hydrogen-rich gas stays within the path of reformer 1˜reformed gas supply pipe 17˜hydrogen electrode 9a of fuel cell 9˜exhaust hydrogen connecting pipe 12. In the event that air flows into this hydrogen-rich gas path due to natural convection from the burner 2a which is open to the atmosphere, there is a fear of explosion of hydrogen.
For this reason, as in this conventional fuel cell electric power generating system, it has been a practice to open the shut-off valve 15 to supply nitrogen as an inert gas from the nitrogen equipment 16 to the path of reformer 1˜reformed gas supply pipe 17˜hydrogen electrode 9a of fuel cell 9˜exhaust hydrogen connecting pipe 12 via the nitrogen supply pipe 14 in stopping the electric power generating operation, thereby purging the hydrogen-rich gas completely and causing it to be burned by the burner 2a. 
In this way the conventional fuel cell electric power generating system prevents explosion of hydrogen by the purging operation with nitrogen thereby to ensure safety.
Such a conventional fuel cell electric power generating system needs to have nitrogen equipment 16, such as a nitrogen cylinder, for the purging operation with nitrogen. In the case where such a system is used in a stationary-type dispersed power generation system for home use or a power source for electric vehicle, there arises a problem that the system requires a large space and an increased initial cost for equipment. There is another problem of an increased running cost because such a nitrogen cylinder needs to be periodically replaced or replenished.